1. Field of the Invention
The invention concerns the manufacture of a reflector having a metallic reflective surface adapted to reflect luminous radiation (in which case the expression "optical mirrors" is commonly used) or non-optical radiation (infrared, etc.) and a metallic matrix composite support extending along the surface.
2. Description of the Prior Art
The use of metallic matrix composite materials for dimensionally stable structures such as optical supports, for example, is well known. These materials have advantages including the following:
the materials are sealed, so that there is no absorption or desorption of moisture, which is beneficial in applications such as space applications where desorption could cause unwanted deformations (or even pollution of sensors); and PA1 the materials are thermally conductive, in particular in the direction of their thickness, which greatly facilitates achieving thermal equilibrium of the structure. PA1 the interface between the support and the reflective surface constitutes a discontinuity in the direction of the thickness of the reflector which can lead to at least localized separation in the event of thermal cycling, for example, or which can degrade the dimensional stability of the support (absorption-desorption in the case of a glue, play in the case of mechanical couplings, etc.); PA1 the geometry of the reflective surface is determined by the geometry of the support and the quality of the process used for attaching the reflective surface, which almost always requires subsequent machining of the surface to meet the shape requirement; and PA1 the mass and the cost of the reflector are higher. PA1 a metallic layer having a reflective surface whose shape is at least approximately identical to the required geometrical shape is disposed on a mold surface having a geometrical shape complementary to the required geometrical shape of the reflector; PA1 fibers adapted to constitute the composite support are draped on the metallic layer, the fibers being metallized by the metallic or intermetallic material adapted to form the metallic matrix; and PA1 the layer and the metallized fibers are subjected to temperature and pressure conditions adapted to press the reflective surface strongly against the mold surface and to cause diffusion welding of the layer with the metallized fibers and of the metallized fibers with themselves so as to integrate the layer to the composite support during consolidation of the support. PA1 the fibers are carbon or graphite fibers; PA1 the fibers are disposed symmetrically on either side of a median surface of symmetry; PA1 the fibers are draped onto the metallic layer in the free state relative to each other; PA1 the fibers are divided into an even number of layers disposed symmetrically to either side of a median surface of symmetry; PA1 the metallized fibers are prepared by vapor phase physical deposition of a layer of metallization onto the fibers so that the metallized fibers are flexible; PA1 the fibers are stranded, rather than free; PA1 the stranded metallized fibers are metallized by dipping into a bath of molten metallic material or by infiltration; PA1 in an alternative embodiment, the fibers are grouped in plates in which the fibers have one, two or three alignment directions; PA1 the plates of metallized fibers having one, two or three alignment directions are obtained by infiltration of the metallic material in the molten state under pressure; PA1 the metallic or intermetallic metallization material is selected from the group of aluminum, aluminum alloy, magnesium, magnesium alloy, copper, copper alloy, titanium, titanium alloy and aluminides, in particular titanium aluminide and nickel aluminide; PA1 the metallic layer is placed on the mold surface in the form of one or more deformable films; PA1 the metallic layer is obtained by preparation of a metallic blank having a blank surface at least approximately identical to the required geometrical shape, the blank being deformable under the temperature and pressure conditions at least in a part of its thickness underlying the blank surface, PA1 the blank is deformable under the temperature and pressure conditions throughout its thickness; PA1 the metallic blank includes a rigid base layer and a coating layer formed of a material that is deformable under the temperature and pressure conditions; PA1 the coating layer is obtained by plasma spraying of one or more metallic powders onto the rigid base layer; PA1 the metallic layer is applied to the surface of the mold by plasma spraying of one or more metallic powders; PA1 the metallic layer includes one or more metallic materials selected from the group of aluminum, aluminum alloy, magnesium, magnesium alloy, copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy and aluminides, in particular titanium aluminide and nickel aluminide; PA1 the reflector is polished; PA1 a further coating is applied to the reflective layer; PA1 the further coating is preferably of gold, applied by vacuum deposition or chemically; and PA1 an oxidation protection layer is deposited on the fibers on the opposite side to the metallic layer. PA1 conforms to the specified shape (without subsequent machining, but possibly with simple polishing); and PA1 is metallurgically attached by diffusion welding to the remainder of the structure (without gluing), enabling the use of materials with coefficients of thermal expansion and Young's moduli substantially different from those of the composite. PA1 a plasma sprayed deposit of metallic powder applied directly to the mold, the nature of the metal being such that it can be diffusion welded to the remainder of the structure; PA1 a metallic blank preformed conventionally to dimensions approximating the specified shape, the material of which is deformable plastically or superplastically and can be diffusion welded to the remainder of the structure under the conditions of consolidation of the metallized fibers; or PA1 a plasma sprayed deposit of metallic powder on the external surface (that adapted to face the mold) of a metallic blank conventionally preformed to dimensions approximating the specified shape, the material of which cannot be deformed plastically under the conditions of consolidation of the metallized fibers; the sprayed metallic powder must be diffusion weldable to the metallic blank during consolidation; likewise the metallic blank and the metallic matrix must be diffusion weldable (in the present instance, the "plasma deposit+preformed blank" combination is placed on the mold, with the "plasma deposit" against the mold). PA1 the metallic or intermetallic materials of the support and the reflective layer are different and their concentrations vary continuously in the direction from the support to the reflective layer and vice versa; alternatively, these metallic materials are identical; PA1 the support is symmetrical about a median surface of symmetry; PA1 the support includes superposed layers of fibers with different orientations in adjacent layers; PA1 the fibers are carbon fibers; PA1 the metallic matrix is formed from one or more materials selected from the group of aluminum, aluminum alloy, magnesium, magnesium alloy, copper, copper alloy, titanium, titanium alloy and aluminides, in particular titanium aluminide and nickel aluminide; PA1 the reflective metallic layer is formed from one or more materials selected from the group of aluminum, aluminum alloy, magnesium, magnesium alloy, copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy and aluminides, in particular titanium aluminide and nickel aluminide; PA1 the matrix has an oxidation protection layer on the side opposite the metallic layer.
These two properties, combined with the fact that the mechanical performance (stiffness and strength) and the coefficients of thermal expansion are beneficial, mean that metallic matrix composites (especially those using light alloys such as aluminum, aluminum alloys, magnesium and magnesium alloys, or copper and copper alloys, which are advantageous because of their thermal conductivity and their ability to withstand high temperatures), and intermetallic matrix composites (including titanium aluminide and nickel aluminide which are advantageous because of their high temperature resistance) are particularly suitable for use in all structures requiring high dimensional stability, such as space telescopes or terrestrial telescopes, for example.
The reflective surface, whether metallic or otherwise, is conventionally fitted to its support by gluing it or by electrolytic deposition subsequent to fabrication of the support, for example.
See for example the article by SULTANA and FORMAN of MIT, Lincoln Lab., Lexington, Mass., USA, entitled "Dimensional stability concerns in the manufacture of graphite/epoxy beam steering mirrors", published in Proceedings of SPIE--The International Society for Optical Engineering, Conference held at San Diego, Calif., USA, 12-13 Jul. 1990--which proposes a laser cavity mirror for space radar including a graphite/epoxy matrix to which an aluminum coating is glued at ambient temperature using an epoxy adhesive.
See also the article by WENDT and MISRA, of MARTIN-MARIETTA ASTRONAUTICS GROUP, DENVER, Colo., USA, entitled "Fabrication of near-net shape graphite-magnesium composite for large mirrors" published in Advances in optical structure systems; Proceedings of the Meeting, Orlando, Fla., Apr. 16-19, 1990 (A91-36651 15-74), Ballingham, Wash., Society of Photo-Optical Instrumentation Engineers, 1990, pp 554-561, which concerns the fabrication of large stable mirrors for space surveillance systems and laser systems including a carbon/magnesium composite support onto which a 127 .mu.m copper layer is deposited.
The disadvantages of attaching the reflective surface to a support already formed include:
An object of the invention is to alleviate the above drawbacks.
The basic idea of the invention is to determine the geometry of the active surface (free surface) of the reflective surface directly and to ensure the greatest possible continuity within the thickness of the reflector between the reflective surface and its support.